Measuring the velocity of a vehicle by a radar device is known for the purpose of traffic monitoring. For this purpose the radar device is stationary at the roadside. The radar device comprises a doppler signal transmitter. A measure for the velocity of the monitored and picked up vehicle may be obtained from frequency shift or doppler shift of the reflected radar beam. A camera is released when a speed limit is exceeded. The picture provided by the camera comprises, conclusively, the picture of the vehicle with its license plate and, reflected into the picture, the indication of the radar device (EP-B-0,286,910; CH-A-662,660).
In common radar devices of this kind a relatively high output frequency occurs which is increased or decreased by the doppler frequency, depending on the direction of the vehicle. Several measurements are taken. The measurements are classified and a histogram is obtained. The shape of the histogram allows conclusions as to eventual disturbance which will make the series of measurements void. With radar devices of this kind a determined minimal relative velocity is required between vehicle and radar device with doppler signal transmitter, of e.g. 20 km/h.
Traffic monitoring equipment comprising radar devices is known for DE-A-37 28 401 (=EP-B-0 304 626=U.S. Pat. No. 4,988,994) in which the radar device is arranged in a moving monitoring vehicle. In this case the relative velocity between the vehicle monitored and the monitoring vehicle is measured. A speedometer supplies the velocity of the monitoring vehicle. The absolute velocity of the vehicle monitored is obtained by addition. When this velocity exceeds a speed limit a photograph is taken. In this known equipment care must be taken that the relative velocity of the vehicles does not fall below a predetermined value.
U.S. Pat. No. 4,335,383 discloses a device for measuring the absolute velocity of a moving vehicle from a monitoring vehicle which also is moving. In this case much effort must be taken for frequency shift in order to suppress doppler signals which occur due to movement of the monitoring vehicle towards ground and surroundings.
Furthermore doppler signal transmitters are known comprising two mixer diodes which are arranged in such a way that they deliver two doppler signals which are dephased against each other by 90.degree.. For the time being the one or the other of these signals leads the other, depending on whether the picked up vehicle approaches or deviates from the doppler signal transmitter. These doppler signal transmitters have, until now, not been used for measuring velocities in traffic monitoring.